The automobile exhaust emission standards legislated in the early 1970s led to crash programs in industry to devise catalytic reactors for oxidizing carbon monoxide and unburned hydrocarbons. 1975 saw the introduction of production model automobiles equipped with catalytic converters. Monolithic catalyst supports and bead-type catalyst supports have been employed in the automotive industry; the design flexibility afforded by the monoliths has been a very important factor in recommending their use in automobile exhaust converters.
Monolithic catalytic supports are continuous, unitary structures consisting of a plurality of small parallel passages running in alignment with the longitudinal axis of the structure and separated by thin walls. Such structures have been termed honeycombs. In some instances the structure will have discontinuities extending transversely through the walls. A multitude of materials has been suggested and tested for use in monolithic support structures; e.g., alumina, alumina-silica, zirconia-alumina, zirconia-magnesia, mullite, zircon, zircon-mullite, titania, spinel, zirconia, Si.sub.3 N.sub.4, and carbon. However, only bodies composed of sintered cordierite (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) or .beta.-spodumene solid solution (Li.sub.2 O.Al.sub.2 O.sub.3.2-8SiO.sub.2) have seen extensive service in that utility.
.beta.-spodumene solid solution exhibits a very low coefficient of thermal expansion, but its maximum long term use temperature of less than 1200.degree. C. severely restricts its applicability in automotive emissions control devices.
Cordierite, or cordierite in combination with a compatible refractory phase, frequently mullite, has comprised the most extensively used material for substrate or support structure for automobile catalytic converters.
In searching for materials demonstrating relatively low coefficients of thermal expansion with higher refractoriness than cordierite, aluminum titanate-based compositions have been developed. Early examples of such products include U.S. Pat. No. 3,549,400, which discloses refractory bodies characterized by grain boundary and intracrystalline cracking having compositions in the Al.sub.2 O.sub.3 --TiO.sub.2 -chrome ore system, and U.S. Pat. No. 3,578,471, which describes refractory bodies also characterized by grain boundary and intracrystalline cracking, but having compositions in the Al.sub.2 O.sub.3 --TiO.sub.2 --MgO system. More recent disclosures include U.S. Pat. No. 4,118,240 describing sintered bodies consisting of aluminum titanate with minor amounts of SnO.sub.2 and SiO.sub.2 ; and U.S. Pat. No. 4,327,188 discussing the fabrication of substrates for catalytic converters from sintered compositions consisting of aluminum titanate and SiO.sub.2 to which Y.sub.2 O.sub.3 and/or La.sub.2 O.sub.3 and/or CeO.sub.2 may optionally and desirably be incorporated. Finally, U.S. application Ser. No. 517,751, filed July 27, 1983 by J. P. Day and I. M. Lachman under the title Aluminum Titanate-Mullite Ceramic Articles, is directed to the formation of sintered articles wherein aluminum titanate and mullite constitute the predominant crystal phases to which minor amounts of Fe.sub.2 O.sub.3 and/or rare earth metal oxides may be included.
The ceramic material comprising the monolithic substrate will exhibit porosity such that the walls of the passages provide sites for the adhesion of the high surface area washcoat which is applied prior to or in concert with the deposit of the catalyst. Furthermore, as was explained in Ser. No. 517,751, sintered ceramic articles containing aluminum titanate as a predominant crystal phase have microstructures with grain boundary and intracrystalline microcracking. Such microcracking enables the articles to yield under thermal stress, thereby significantly improving the thermal shock resistance thereof.
In addition to the choice of starting materials used in forming ceramic bodies, one factor influencing the level of open porosity and the size of the pores is the sintering temperature employed; i.e., higher temperatures will normally lead to lower values of open porosity and smaller pore size. For example, when the compositions of Ser. No. 517,751 are sintered at 1400.degree. C., open porosity will average about 30-45%, but when sintered at 1500.degree. C., porosity drops to about 8-26%. In like manner, the pore size will range from about 1-15 microns depending upon the sintering temperature utilized. In general, the preferred open porosity interval is between about 25-45% with pore sizes between about 5-15 microns.
As can well be appreciated, maximum benefit of catalytic activity will be attained when the catalyst is deposited upon a substrate of very high surface area, such that the greatest number of sites will be available for contact with the fluid passing thereover. In general, the ceramic monolithic supports have surface areas of only about 0.1-1 meter.sup.2 /gram (m.sup.2 /g). Therefore, coatings exhibiting very high surface areas (frequently about 50-200 m.sup.2 /g) have commonly been applied to the supports in amounts of about 5-20% by weight of the substrate. Those coatings, termed "washcoats" in the industry, most frequently consist principally of aqueous slurries of gamma-Al.sub.2 O.sub.3. Immersion in an aqueous slurry or the use of an injection process comprise the simplest methods for coating the monolithic support. The coated substrate is at least partially dried and heated to a temperature sufficient to bind the Al.sub.2 O.sub.3 -containing particles together. Thereafter, the support is impregnated through an immersion or injection process with a solution of a base metal and/or noble metal catalyst. Currently in the automotive field, one or more of the noble metals palladium, platinum, and rhodium is utilized as the catalyst. Hence, in common practice a solution is prepared of a thermally decomposable compound of at least one of those metals, e.g., H.sub.2 PdCl.sub.6, H.sub.2 PtCl.sub.6, and H.sub.2 RhCl.sub.6. After at least partial drying, the coated structure is fired to a temperature sufficient to thermally decompose those compounds to metallic particles.
Whereas the customary procedure contemplates serially applying the washcoat and catalyst, as described immediately above, it is also possible to mix the catalyst solution with the washcoat and then impregnate the substrate with the combination. That practice eliminates one coating step, but there has been some question as to the equivalent uniformity of catalyst available for reaction.
As has been observed above, sintered structures containing aluminum titanate as the predominant crystal phase evidence microstructures exhibiting grain boundary and intracrystalline microcracking. That phenomenon enhances the utility of such compositions since it greatly improves the thermal shock resistance of bodies fabricated therefrom. However, when monolithic catalytic supports were prepared in accordance with the procedure outlined above, cracking and breakage thereof resulted when they were plunged into a furnace operating at about 500.degree. C., that being a standard test for thermal shock resistance. The coefficient of thermal expansion was measured on the defective products and discovered to be about 45.times.10.sup.-7 /.degree.C. over the range of room temperature (R.T.)-1000.degree. C., whereas the initial, uncoated body exhibited a coefficient of thermal expansion of less than about 5.times.10.sup.-7 /.degree.C. over the range of R.T.-1000.degree. C. This dramatic increase in thermal expansion is believed to be due to two phenomena occurring from the Al.sub.2 O.sub.3 -containing washcoat penetrating into the microcracks. Substantial thermal strains are developed because the crystals of the support cannot expand into the microcracks when the support is heated. Furthermore, the washcoat material demonstrates a high coefficient of thermal expansion, normally in excess of about 75.times.10.sup.-7 /.degree.C. (R.T.-1000.degree. C.). The effect of these two factors is to sharply raise the overall expansion of the body.
Accordingly, the principal objective of the instant invention is to provide means for preventing the entry of the washcoat into the microcracks of a sintered ceramic substrate.